The presence of organic complexants, such as gluconic acid, in an intermediate-level radioactive-waste (ILW) repository may have a detrimental effect on the sorption of radionuclides, by forming organic complexes in solution. In order to assess this, stability constants are required for the complexes formed with radionuclides at high pH. This study reports the stability constants for the reactions of some divalent metals with gluconic acid (Gl). The metals studied were Cd, Co, Fe(II), and U(VI) at pH 13.3; and Co and U(VI) at pH 7. The constants were measured by the Schubert (ion-exchange) or solubility product methods. Stoichiometries of the complexes were also determined. At pH 7 each complex was of the form M1Gl1, with log β values suggestive of salt formation. At high pH, log β values were between 13 and 20. The constants have enabled speciation calculations to be performed showing the effect of gluconic acid on the metal's solubility, with data for Ni included. Solubility is predicted to increase in the presence of Gl from pH 9 to 13.5, suggesting that it may have an impact on radionuclide behaviour. The largest solubility increases are for Cd and Co, the smallest, U(VI).